Thermoplastic polyesters based on terephthalic acid and diols such as ethylene glycol and 1,4-cyclohexanedimethanol have proven to be very desirable for injection molding articles for high strength applications. These polymers show a desirable balance of mechanical, thermal, electrical and rheological properties. However, the use of these polymers is limited where high impact strength at low temperatures is required, especially at temperatures near 0.degree. C. It is, therefore, highly desirable to improve the low temperature impact strength of polyester blends used for injection molding.
Thermoplastic compositions exhibiting high impact strength are found in U.S. Pat. Nos. 3,562,200, 3,580,965 and 4,172,859. U.S. Pat. No. 3,562,200 discloses a thermoplastic composition containing polyethylene terephthalate having finely dispersed discrete particles of an amorphous or poorly crystalline high molecular weight copolymer of ethylene and a carboxylic acid ester. In contrast, the ethylene copolymers of the present invention have a high, degree of crystallinity.
U.S. Pat. No. 3,580,965 discloses thermoplastic molding compositions containing linear, saturated polyesters and 0.05 to 20 weight percent of copolymers of .alpha.-olefins and vinyl esters of saturated monocarboxylic acids. While the use of polyethylene terephthalate and polycyclohexane-1,4-dimethylol terephthalate polyesters are disclosed in U.S. Pat. No. 3,580,965, such patent does not suggest the possibility of obtaining unexpected results from a polyester containing a combination of ethylene glycol and 1,4-cyclohexanedimethanol. In fact, the present inventors have discovered that the notched Izod impact strength at 0.degree. C. of neat polyethylene terephthalate is not effectively improved by the addition of copolymers of ethylene with vinyl acetate.
U.S. Pat. No. 4,172,859 discloses multi phase thermoplastic compositions containing a polyester, a copolyester with polycarbonate and at least one other phase containing a wide range of copolymers. Copolymers of ethylene and vinyl acetate, which are necessary in the present invention, are excluded from a long list of useful copolymers for toughening polyester in U.S. Pat. No. 4,172,859. In such patent, ethylene and vinyl acetate are only present in the form of a terpolymer with, for example, carbon monoxide. In addition, U.S. Pat. No. 4,172,859 states that the particle size of the copolymers must be in the range of 0.01 to 3.0 microns to achieve high impact strength. In contrast, the particle size of the copolymers of the present invention approach co continuous phases.